The signs and symptoms of large bowel obstruction are:

Choice A reason:
Arterial emboli from a heart valve are not the most common cause of myocardial ischemia. While emboli can obstruct blood flow and cause ischemia, they are a less common cause compared to atherosclerosis. Embolic events typically result from conditions like atrial fibrillation or valvular heart disease but do not account for the majority of myocardial ischemia cases.

Choice B reason:
Atherosclerosis is the most common cause of myocardial ischemia. It involves the buildup of plaque within the coronary arteries, leading to reduced blood flow and oxygen supply to the heart muscle. This condition is a leading cause of coronary artery disease and is responsible for the majority of myocardial ischemia cases.

Choice C reason:
Idiopathic vasospasm, although a cause of myocardial ischemia, is not as common as atherosclerosis. Vasospasm refers to the sudden constriction of a coronary artery, which can reduce blood flow to the heart muscle. However, it accounts for a smaller percentage of ischemia cases compared to the widespread prevalence of atherosclerosis.

Choice D reason:
Venous emboli are typically associated with conditions such as deep vein thrombosis and pulmonary embolism, affecting the venous circulation rather than the coronary arteries. They do not commonly cause myocardial ischemia, which primarily involves the coronary arteries and arterial blockages.

Choice A reason:
Hypotension is not the likely cause of chest pain that does not improve after sublingual Nitroglycerin in a patient with a history of atherosclerosis. While hypotension can cause symptoms, the description of chest pain at rest that does not respond to Nitroglycerin suggests a more severe cardiac issue.

Choice B reason:
Stable angina typically occurs with exertion and is relieved by rest or Nitroglycerin. Chest pain consistent at rest that does not improve with Nitroglycerin suggests unstable angina, which is a more serious condition and requires immediate medical attention.

Choice C reason:
Unstable angina is the correct answer. It occurs when chest pain happens at rest and is not relieved by Nitroglycerin, indicating that the blood flow to the heart is severely restricted. This condition is a medical emergency as it can lead to a myocardial infarction.

Choice D reason:
Coronary vasodilation would typically relieve chest pain by increasing blood flow to the heart. The pain described is more consistent with unstable angina, where there is a significant reduction in blood flow to the heart that does not respond to vasodilation.